It is common for a hydraulic shock absorber to include an inner housing portion which is slidably coupled to an outer housing portion such that the effective length of the shock absorber is variable. The inner and outer housing portions together define an internal cavity or chamber, which contains shock absorber fluid. The inner housing portion is known in the art as a ‘slider’, ‘sliding tube’, ‘inner cylinder’, or ‘piston’, and the outer housing portion is known as a ‘main fitting’, or ‘outer cylinder’.
The region where the inner and outer housing portions overlap defines an annulus between adjacent surfaces of the inner and outer housing portions, which varies in size in accordance with the extension state of the shock absorber.
One or more dynamic seals are generally provided within the annulus to confine the shock absorber fluid to the chamber. The dynamic seals can be mounted on an inner face of an annular ring, which is inserted into and fixed in place within the annulus such that the dynamic seals press against the inner housing portion as the shock absorber extends and retracts, inhibiting the passage of shock absorber fluid from the chamber to the outside environment. One or more static seals can be provided on an outer face of the annular ring to bear against the outer housing portion when the annular ring is fitted within the annulus.
The effectiveness of a dynamic seal in terms of inhibiting the passage of shock absorber fluid is dependent on the force with which it is biased against the inner shock absorber portion. However, a strong biasing force results in a high level of wear. Therefore, there exists a trade-off between on the one hand the effectiveness of the fluid barrier and on the other hand the lifespan of the dynamic seal. Consequently, it is common for shock absorber fluid to leak via dynamic seals, particularly when a shock absorber remains in a static condition for a prolonged period to time.